The present invention relates to a continuously variable valve actuation apparatus in an internal combustion engine which has charge exchange valves. The apparatus is able to continuously vary stroke, lifting duration, and phase.
Continuously variable valve actuation systems are known in the art, and disclosures of mechanical, hydraulic, and electromagnetic systems are known. The advantages of such systems are numerous. Most importantly, such systems can control the charge filling of a four-cycle spark ignition engine without the conventional throttle valve, thus reducing the pumping loss and improving the efficiency. Another advantage is the ability to generate valve lift curves that suit a wide range of operating conditions. The following discussion of prior art will focus on mechanical systems that presently appear to be relevant for the invention being disclosed here.
A first type of apparatus is disclosed by U.S. Pat. No. 6,029,618 (Nara et al.) This apparatus consists of an eccentric crank and a rocking cam placed coaxially on a driveshaft, and operatively connected by a link mechanism that can vary stroke and lifting duration of the valves. U.S. Pat. No. 6,390,041 (Nakamura et al.) points out that this system is sensitive to dimensional errors of the components when it operates in the region of low lift. This incurs higher requirements for machining accuracy and hence higher production cost. The latter patent discloses a microcomputer-based controller to help compensate for the problem, but it does not solve the fundamental mechanical issue.
A second type of apparatus is disclosed by U.S. Pat. No. 6,425,357 (Shimizu et al.) This apparatus consists of a rocker assembly inserted between a camshaft and a follower connected to a valve of the engine. The rocker assembly includes an input portion driven by the cam, and an output portion contacting the follower. By varying the angle between the input and output portions with a sliding gear, stroke and lifting duration can be varied. U.S. Pat. No. 6,823,826 (Sugiura et al.) points out that the sliding gear is difficult and expensive to manufacture, as it is difficult to machine the inner parts of the helical splines for the sliding gear, and as it must be machined with high accuracy, in order to avoid a situation where only a small fraction of the teeth carry the entire load. The solution devised by this patent is to replace the sliding gear with a single pin, contacting a diagonal hole or a diagonal surface. This will, however, shift the entire load of the sliding gear to a single point of contact. US Pat. Application Pub. No. 2007/0163523 (Miyazato et al.) points out another problem with this type of apparatus. If the engine block is manufactured from an aluminum alloy, and the control shaft for moving the sliding gear is manufactured from steel, then the different temperature expansion coefficients of these materials will cause a difference between the adjustment among the cylinders in the engine block. This hinders accurate control of valve lift. The devised solution is to manufacture the engaging parts of the control shaft (the sliding gear) from steel, and the parts in between from an aluminum alloy. This solution will add to the manufacturing cost and complexity of the system as well. Finally, this type of apparatus is adding to the total spring load supported inertia of the valve train, thus limiting the maximum operation speed and increasing friction due to the requirement for higher spring loads.
A third type of apparatus is disclosed by U.S. Pat. No. 6,907,852 (Schleusener et al.) This apparatus consists of a spring loaded pivoting lever, the top end having a roller contacting a control path and an eccentric control shaft which controls the position where the roller contacts the path. The camshaft presses against another roller in the middle of the lever to rock it sideward. At the other end of the lever, a ramp contacts a follower connected to the valve. The eccentric control shaft is rotated by an electric stepper motor, varying stroke and lifting duration. This type of apparatus is also adding to the total spring load supported inertia of the valve train, thus limiting the maximum operation speed and increasing friction due to the requirement for higher spring loads. Moreover, the requirement for space in the engine head is high.
Common for all three types of apparatuses above is the requirement for a separate additional device to vary the phase. This adds to the total cost and complexity of embodiments, and introduces the extra complication of synchronizing the two systems, especially during transient states. In the worst case, poor synchronization can cause a valve to collide with a piston top, resulting in major damage to the engine. U.S. Pat. No. 6,820,579 (Kawamura et al.) discloses an electronic control system for synchronizing the two systems, exemplified by the first type of apparatus above. This system accounts for the rate constants of change for the respective systems during transient states. U.S. Pat. No. 7,434,553 (Nakano) discloses a combined hydraulic circuit for controlling the two systems in the case of the second type of apparatus above. These extra control systems also add to the cost and complexity of embodiments.
Examples of apparatuses which can vary valve stroke, lifting duration, and phase simultaneously are disclosed in U.S. Pat. No. 6,968,819 (Fujii et al.) and U.S. Pat. No. 7,299,775 (Tateno et al.). Common properties for these inventions are that they add to the spring load supported inertia of the valve train, and that they are relatively complicated mechanisms. Presently, no apparatus which can vary stroke, lifting duration, and phase simultaneously is known to be in mass production.